Dark Ankous are randomly and uncommonly summoned during the fight with Skotizo. If the player kills a Dark Ankou, Skotizo may randomly summon another. There is a limit of one Dark Ankou in the room at a time. If a Dark Ankou spawns on the opposite side of Skotizo from the player, or if the player is attacking an altar, it will wander around the room. If the Dark Ankou walks near the player and they are attacking Skotizo, it will become aggressive towards them.
There is no real benefit fighting Dark Ankous, so the player should instead focus on fighting Skotizo.